1. Field of Invention
This invention generally relates to professional audio production and electronic musical instruments, specifically to the interfacing of electronic audio signals between what is commonly known among audio engineers as instrument-level audio devices and professional-level audio devices; relating to the connection between various electronic musical instruments or devices to and from various professional audio signal processing or recording devices.
2. Prior Art
The worlds of the professional audio engineer and the professional musician come together in the recording studio and also in the performance venue or on-stage. However, professional audio equipment designed for the audio engineer uses what is known in the industry as professional-level interfacing which came from the early telephone industry standards for audio electronics, whereas instrument-level audio equipment is designed to use what is known to the audio engineer as instrument-level interfacing which evolved separately around the high impedance pick-ups and amplifiers used for many electric musical instruments.
The telephone industry 0 dBm reference for measuring audio signal power levels was adopted by the early audio engineer. Many modern professional-level interfaces are primarily concerned with voltage level and not power level; to delineate this, the professional audio industry also uses 0 dBu (a variation of 0 dBm). Professional audio signal levels are generally between 0 dBu to positive 20 dBu, whereas instrument audio signal levels are generally between negative 20 dBu and 0 dBu. Also, professional audio interfaces use lower impedances which are better for connections at a distance and are less susceptible to interference and pick-up of undesirable noise.
Electric musical instruments use a transducer, such as a magnetic pick-up device, mounted on or inside a musical instrument. The nature of these pick-up devices requires that they have higher output impedances in order to be sensitive enough to produce a useable electronic voltage signal. However, when these higher impedance pick-up devices are connected directly to lower impedance professional audio equipment, the higher output impedance forms a voltage-divider with the lower input impedance. This causes loss and degradation of the original signal. Therefore, these electronic pick-up devices require specialized high input impedance pre-amplifiers which minimize the voltage-divider effect in order to preserve the original audio signal; thus instrument-level audio equipment evolved to accommodate lower-level, higher-impedance signals.
Interfacing these instrument-level signals with professional audio devices has been a challenge to both the audio engineer and professional musician. Oftentimes the musician's performance is influenced and inspired by the sound and volume of their audio processing devices. For example, the volume of the musician's amplifier may be used to create sustain in the musician's performance by intentionally generating positive-feedback. For the audio engineer, this means that the original signal from the pick-up device must be connected through the musician's audio processing devices. This is essential to the musician's performance but at odds with the goal of the audio engineer which is to capture the purest audio signal directly from the pick-up without adding the characteristics of the musician's audio processing devices. This is due to the fact that the audio engineer may wish to experiment with a variety of audio processing devices and may choose not to employ the musician's audio processing devices. A well known compromise has been to employ a signal splitting device known as a direct-box which can be used to split the signal from the instrument to allow a part of the original signal to be sent to the musician's audio processing devices and a part of the original signal to be sent to the audio engineer. However, it has been difficult to split the original signal without affecting its quality. Musicians and audio engineers both complain that signal splitting devices have undesirable side-effects and disadvantages, for example:                (a) They are known to introduce some unacceptable tonal-coloration. The ideal signal splitting device should preserve the original audio signal in the purest form and not add any hint of its own character to the signal.        (b) They add some unwanted noise. The ideal signal splitting device should add as little of its own noise as possible. Once noise is added to the original signal it is difficult to remove without also removing some of the desired signal.        (c) They introduce some signal distortion. The ideal signal splitting device should not distort the original signal unnecessarily.        
An improved signal splitting device would need to have a very high input impedance to interface with the original signal from the pick-up device. The signal splitting device would need to preserve the lower-level higher-impedance signal for interfacing with the musician's instrument-level audio processing devices. Additionally, the signal splitting device would need to boost the audio level and lower the signal impedance to interface with professional-level audio processing devices.
Also, a musician may want to use a professional-level audio device in-line with their instrument-level audio processing devices. To do this, the musician would need an interface to insert the professional-level audio devices into his instrument-level signal chain. Previously, the musician would start with an interface device such as the discussed direct-box to raise the level and lower the impedance of the audio signal coming from his instrument-level audio processing device for interfacing to a professional-level audio device. After processing the signal through the professional-level audio device the musician would then need a reverse interface device to lower the level and raise the impedance of the audio signal returning from the professional-level audio processing device for interfacing to the remainder of his instrument-level audio devices.
Also, an audio engineer may want to re-process a professional-level audio signal through instrument-level audio processing devices to achieve a particular effect. To do this, the audio engineer would need an interface to insert the instrument-level audio processing devices into his professional-level audio signal chain. Previously, the audio engineer would start with a reverse interface device to lower the level and raise the impedance of the professional-level audio signal for interfacing to the instrument-level audio processing devices. Then the audio engineer would use an interface device such as the previously discussed direct-box to raise the level and lower the impedance of the audio signal returning from the instrument-level audio processing device for interfacing to his professional-level audio devices. In some instances, it may be desirable to intentionally mismatch input and output impedances to achieve a desired effect or to simulate a tailored vintage sound, thus a way to make the impedances variable would be needed.
Previously, examples of devices that could be employed to act as a reverse interface device for the purpose of interfacing a signal coming from a professional-level audio device to an instrument-level device as well as similar devices which employ the same principles, have several limitations and disadvantages, for example:                (a) These devices generally use a transformer, vacuum tube or FET means, all of which are known to introduce some undesired tonal-coloration. The ideal reverse interface device should preserve the original audio signal in the purest form and not add any hint of its own character to the signal.        (b) Also, because these devices use a transformer, vacuum tube or FET means, they introduce some undesirable noise and signal distortion. The ideal reverse interface device should not distort the original signal unnecessarily.        (c) A well known re-amplification device employs a potentiometer which simultaneously adjusts the output level and the output impedance which makes it difficult (in some cases impossible) to match the level and the impedance as desired. The ideal reverse interface should allow level and impedance to be set independent of each other.        (d) A well known re-amplification device which is passive must be physically located near the instrument-level audio devices; it cannot be remotely located in a convenient location such as in an equipment rack inside a recording control room.        (e) Because a well known re-amplification device which is passive uses no power it cannot provide active signal metering.        (f) Also, because a well known re-amplification device which is passive uses no power, it cannot provide illumination which is desired by the professional audio engineer who often works in a darkened environment, such as a theater for example.        
3. Objects and Advantages
Accordingly, I have invented an improved bidirectional, variable-impedance instrument-level to professional-level audio interface with signal splitters and balanced DC power with the following objects and advantages:                Extremely high audio quality. This device adds virtually no undesirable signal-coloration, distortion or noise. This device has a new novel circuit design for its internal power conditioning which produces a quality of specifications not achieved in previous interface devices.        An interface device with a signal splitting feature having variable input impedance for interfacing with instrument-level pick-up devices or instrument-level audio processing devices. The signal splitting device provides five outputs. Two outputs which simply buffer the original signal. A third and fourth output which provide variable output level and variable output impedance. A fifth output which provides a variable output level and professional low impedance balanced interface.        A reverse interface device with a signal splitting feature having a low impedance input for interfacing with professional-level audio equipment. The signal splitting device provides two outputs. Both outputs provide variable output level and variable output impedance.Other Objects and Advantages are:        Provision of variable-impedance inputs and outputs for flexible interfacing to a variety of common audio equipment.        Provision of a reconfigurable signal ground reference to help solve common grounding problems.        Provision of signal level indication to help ensure levels are set within an acceptable range.        Provision of illuminated rear connector labels to make connections visible in poor lighting situations common in recording studios and back-stage environments.        
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.